Masts to support objects at a height or distance from a base are widely used. Common examples are antennas, and supports for instruments and solar panel packages. The fields of application of these masts are wide and diverse. Communication systems in the field is one example. The support of instrument packages on a space vehicle is another.
Where it is feasible to provide the mast as a permanent installation to be assembled in place from separate components, there is little difficulty. However, when the mast must be conveyed in an already assembled condition to some selected place to be installed, a conventional pre-assembly procedure is not acceptable. Battlefield antennas, and spacecraft installations where the erected antenna will project beyond the spacecraft structure, are examples of situations where time and circumstances do not permit a leisurely assembly or where a pre-assembled device cannot be employed because it exceeds the dimensions which can conveniently be transported, or which in use projects beyond allowable limits. For such situations, the mast must not only be pre-assembled, but also must be stored in a retracted condition in an envelope of much-reduced dimensions and volume. For example, a ratio of 50:1 for the deployed length of a mast relative to its retracted, stored condition, is a conventional objective.
A 50 foot mast cannot readily be carried around a battlefield, and certainly cannot project from a space vehicle at launch time. The same mast, packed into a cannister can be carried around a battlefield and quickly be set up, or can be packed in a cannister in a space vehicle so it can tolerate launch forces, and later in weightless space be erected. Deployable masts, including triangular lattice masts, are well-known. A conventional example is shown in James E. Webb U.S. Pat. No. 3,486,279, issued Dec. 30, 1969 which is incorporated herein by reference in its entirety for its showing of the basic structure of a mast according to this invention.
The capacity of a mast to be deployable is well-shown in the referenced Webb patent, which is suitable for a wide range of mast applications. For example, assume that a non-directional antenna or instrument package is to be supported. Then any position of a supported load or structure is acceptable. An example is a magnetometer, which requires no specific directional orientation.
However, consider a solar panel whose surfaces must be specifically oriented relative to the sun, or a directional antenna which must be directed toward a specific point. Then, because the basic support structure such as a space vehicle cannot be expected to be oriented for the benefit of only one of its systems, then the system itself must be oriented. A basic feature of this orientation is rotatability of the deployed mast around its axis. This is a requirement for many, even most, deployable mast systems, and this objective has in fact extensively been commented on, and has been provided for.
There is an obvious "brute force" solution to this requirement. It is to provide a cannister for the storage and later support of an extensible mast, and then to provide means to rotate the cannister. This has in fact been done. While it fulfills the requirements for retraction, deployment and rotation, it requires two sets of controls and drive means-one for retraction and deployment, and another for rotation of the entire assembly.
Such an arrangement would not be intolerable for a fixed installation on earth which did not require portability or minimization of weight. However, where weight, bulk, circuitry and complexity of structure should be minimized, a dual drive involves penalties that ought to be averted. The basic problem is that reversing the direction of drive also retracts the mast, and vice versa. This precludes the rotation of the deployed mast independently of a retraction or deployment movement. For this reason, independent drive systems are provided for deployment/retraction and for rotation. Because masts of this type are used in situations regarded as portable, these are very costly penalties. Additional throw-weight on space vehicles is astonishingly expensive. Additional weight for a device to be carried on the battlefield can be deadly.
It is an object of this invention to provide a triangular lattice structure deployable to form a mast, and retractable to a helical configuration, together with means to enable it to be rotated independently of its supporting structure by the same drive mechanism as is used to deploy and to retract it.
In considering this invention, it should be borne in mind that the mast when erected is a triangular lattice structure which when deployed is quite rigid and self-shape retaining. It constitutes three longerons located at the apices of an equilateral triangular cross-section. These longerons are made of unidirectional fiberglass rods which are stiffly flexible and inherently tend to return to a straight line shape. They are, however, flexible enough to be coiled into a cylindrical cannister in a helical shape. A batten frame is formed of three batten members which extend between respective pairs of apices. Their dimensions are such that, when the structure is erected, they are bowed in an Euler type deflection which maintains a separative force between the longerons. Stability and the Euler forces are maintained by a group of flexible diagonal members whose essential properties are tensile strength and compressive collapse.
Such a structure when employed has remarkable columnar properties, but it can also be coiled into a cylindrical cannister. When rotated by drive means it passes through a transition section which converts the shape from a helix to a prismatic column or vice versa.
This is not as simple as it sounds, because these mast constructions have peculiarities of their own, depending on the flexural properties of the longerons and the batten members, and their relative dimensions. For example, in some relationships, unless provision is made in the transition section to avert it, the mast can emerge not as a prismatic structure, but as a stable twisted structure. This must be prevented.
Whatever the situation, the purpose is to erect from a stored coiled, helical structure, a triangular lattice mast with significant resistance to bending failure and columnar collapse, and in addition to enable the deployed mast to be rotated by the same drive means as is used to deploy and to retract the mast.